The present invention relates to a dielectric filter using coaxial resonators in which an electrode is formed on inner peripheral surfaces and outer peripheral surfaces of a parallelepiped-shaped dielectric member through which holes are formed in an axial direction, and it relates more particularly to a structure of a connector terminal associated with each coaxial resonator for input-output of a signal.
FIG. 11 is an exploded perspective view showing a primary part of a conventional dielectric filter.
The conventional dielectric bandpass filter 100 has a parallelepiped shape having a laterally extending rectangular cross section. The dielectric filter 100 is composed of a filter unit 101 having an electrode 104 (indicated by dots in FIG. 11) made of silver, copper or the like formed on outer peripheral surfaces of a parallelepiped dielectric member 102, which has five parallel through-holes 103 each extending in the axial direction, with the electrode 104 further extending onto inner peripheral surfaces of the respective through-holes 103; a plurality of connector terminals 105 for input-output of signals to and from the filter unit 101; and a metal case 106 for mounting the filter unit 101 onto a printed substrate (not shown).
The filter unit 101 is integrally made of filter blocks comprising five longitudinally connected coaxial resonators, terminals of which are short-circuited with each other. The electrode 104 formed in the inner peripheral surface of each through-hole 103 of the dielectric member 102 constitutes an inner conductive member of each coaxial resonator. The electrode 104 formed on the outer peripheral surface that is parallel with the through-hole 103 of the dielectric member 102 constitutes an outer conductive member of each coaxial resonator. The outer and inner conductive members are short-circuited by the electrode 104 that is formed on one end face 102B of the dielectric member 102.
Each connector terminal 105 is composed of an insert member 105A made of insulating material such as polymethylpentene and a pin 105B obtained by plating a surface of a steel wire such as brass with soldering material. The insert member 105A has the same cross-section as that of the above-described through-holes 103 with a flange portion 105C being formed at its proximal end portion. The flange portion is used to prevent the insert member 105A from being fully inserted into the through-hole 103.
The insert member 105A of each connector terminal 105 is inserted by pressing into the associated through-hole 103 of the dielectric member 102 so that the proximal portion of the pin 105B is supported on the axis of the through-hole 103.
The metal case 106 is used for grounding the filter and for preventing the connector terminals 105 from being pulled apart from the through-holes 103 upon actually mounting the dielectric filter 100 onto the printed substrate. The end portion of the filter unit 101, on the side of the open end face 102A, is inserted between a pair of mounting plates 106A, and the mounting plates 106A are attached and fixed to the outer conductive electrode 104 by soldering or the like.
In the conventional dielectric filter 100, since the flange portion 105C is provided at the proximal end portion of each insert member 105A and the flange portion 105C is arranged between the open end face 102A of the dielectric member 102 and the metal case 106 for preventing the insert members 105A from being pulled apart from the through-holes 103 and from being fully inserted into the through-holes 103, it is necessary to use the metal case 106, which leads to an increased number of assembly steps and an increased cost.
Also, if, in order to reduce the actual mounting area required, an arrangement is adopted in which the dielectric filter 100 is mounted on the printed substrate with the open end face 102A of the filter unit 101 facing the actual mounting surface of the printed substrate, the above-described flange portions 105C are interposed between the open end face 102A of the filter unit 101 and the actual mounting area of the printed substrate, which then leads to unstable mounting of the dielectric filter 100.